1. Field of the Invention
The present invention relates to a microprocessor capable of carrying out instructions having different data lengths.
2. Description of the Related Art
Generally, a microcomputer is constructed by a central processing unit (CPU), a memory and an input/output interface. Also, the CPU, called a microprocessor, is formed by an instruction decoder, registers, operational circuits such as an arithmetic and logic unit (ALU) and shifters.
As semiconductor technology has been developed, the data length of microprocessors has been increased from 4 bits via 8 bits, 16 bits and 32 bits to 64 bits. Note that, generally, as the data length has been increased, the power dissipation of microprocessors has also been increased.
On the other hand, microprocessors have been introduced into portable electronic apparatuses powered by batteries, and as a result, the reduction of power dissipation is required.
In a prior art 32-bit microprocessor capable of carrying out an 8-bit data length instruction, a 16-bit data length instruction and a 32-bit data length instruction, 32-bit registers and 32-bit operational circuits are always operated. As a result, it is impossible to reduce the power dissipation. This will be explained later in detail.
Similarly, in a prior art 64-bit microprocessor capable of carrying out an 8-bit data length instruction, a 16-bit data length instruction, a 32-bit data length instruction and a 64-bit data length instruction, 64-bit registers and 64-bit operational circuits are always operated. As a result, it is also impossible to reduce the power dissipation.